Study On Fluid-Structure Interactional Load And Trajectory Characteristics Of High-speed Water Entry Of Projectile

High-speed water entry problem has been concerned for a long time due to comprehensive nonlinearity,the coupling of gas-vapor-liquid-solid multiphase flow,the randomness of flow field,and so on.Structural response loads and waterborne ballistic control in high-speed water inflow problems have always been studied and widely discussed by scholars from various countries.In the process of high-speed projectile entering water,it's easy to be damaged for the sensitive components of the projectile due to the structural bending moment and axial impact at the instant of slamming.During the underwater navigation,because of the existence of the phenomenon of flapping and tail flap,etc.,the radial load may cause bending failure of the projectile structure.It also has been concerned that what the underwater trajectory of the projectile is and the active control of the projectile.This series of problems have been focused by people in the process of water entry.In this paper,the arbitrary Euler-Lagrange(ALE)analysis method has been applied to simulate the high-speed water entry process with the help of LS-DYNA software.The effect of projectile head shape,projectile body mass center position,initial velocity and tail thrust on the structural response load of projectile body at different incident angles is discussed,which provides a reference for the structural design and attitude control of the high-speed water-injecting vehicle.The preliminary design of the projectile structure of the high-speed water entry is carried out based on the existing picture data.The ALE method has been applied to simulate the process of this fluid-structure interaction problem.Numerical results including velocity decay vs.time of fragment,cavity evolution for different impact velocities,are quite similar to experimental tests.The reliability and accuracy of ALE method based on LS-DYNA softwareare verified.The structural response load in the process of the high-speed water entry of the projectile is discussed.The impact acceleration of the whole structure,the axial impact interface force of the projectile and the bending moment load of the cross section of the projectile are analyzed The vertical water entry with a pitch angle of 90° and the oblique water entry with a pitch angle of 45° have been simulated.The effects of the head shape,centroid position and initial velocity on the structural response load are summarized and studied.The evolution of cavitation and the ballistic law of trajectory in the process of projectile entering water at high speed are studied.The typical phenomena appearing in the process of water entry are described in detail.The coupling effects of ballistic flapping characteristics,the position of the center of mass,the initial velocity and the cavitation evolution on the projectile trajectory are also analyzed in this paper.As the comparison of the depth of the water entry zone and the speed of the missile's flapping,the high-speed underwater trajectory of the projectile is summarized.Finally,the simulation analysis of the water entry process with active thrust is completed The phenomenon of "jumping projectile" existing in the process of water entry at a small angle is analyzed.The typical load variation and the phenomenon of free surface splash are explained.For the complex hydrodynamic fluid-structure interaction process of the high-speed projectile carrying the tail thrust entering the water,the variation of the structural response load with different initial velocity and the ballistic flapping movement are discussed.The influence of the existence of tail thrust on the water entry process is summarized.